We are currently analyzing the coexistence of calretinin (CR), a calcium binding protein, and tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of dopamine, in the A11-A12-A13 dopamine containing neurons in rat brains. We used double-labelling immuofluorescence on cryostat brain sections to distinguish three distinct cell types: cells immunoreactive for CR only (CR+), cells immunoreactive for TH only (TH+) and cells in which both proteins were localized (CR+TH). CR+TH cells were found in both the A11 and A13 cells in the hypothalamus (A13) and midbrain (A11). In both nuclei individually labeled cells were also detected. No colocalization was observed in the A12 (arcuate) nucleus. The presence of CR in dopaminergic cells is reminiscent of the substantia nigra cells (A9/A10) which also contain a population of cells that contain CR. The function of the A11/A13 dopamine-containing cells is unknown.